The present invention relates to a differential device adapted to transmit power through a fluid and, more particularly, to a differential device of the type using a viscous fluid coupling mechanism.
A differential device installed in a power transmission system of a motor vehicle and others has traditionally been implemented with planetary gears. A recent achievement in the realm of differential devices is a device which uses a viscous fluid coupling mechanism, as disclosed in European Laid-Open Patent Publication No. 68309. This type of differential device is attracting increasing attention due to its desirable performance.
A differential device using a fluid as mentioned above includes a differential case which is driven by a power unit, and a pair of rotary output shafts which are individually coupled to the differential case by viscous fluid coupling mechanisms. Specifically, in a power transmission chamber defined in the differential case, a plurality of drive plates which are rotatable integrally with the case and a plurality of driven plates which are rotatable integrally with their associated output shafts are arranged alternately overlapping each other. A viscous fluid such as silicone oil is confined in the case. In this construction, as the differential case is rotated, the drive plates in the power transmission chamber are rotated while, at the same time, the fluid transmits the rotation of the drive plates to the driven plates due to its viscosity, thereby rotating the output shafts. The difference in rotation speed between the output shafts is accommodated by the slippage of the fluid.
Generally, in a differential device of the type described, a pair of coaxial output shafts have inner end portions which are positioned within the differential case in a face-to-face relationship. Driven plates are directly mounted on the end portion of each of the output shafts which are disposed in the power transmission chamber. A problem with this configuration is that the differential device has to be assembled together with the output shafts. Specifically, because the drive plates and the driven plates are arranged alternately with each other, the respective plates have to be mounted in the case after the case has been mounted on the output shafts; it is impossible for the differential device to be constructed as a unit.
A prerequisite with this type of differential device is that a small clearance be defined between inner end faces of the output shafts to allow the output shafts to rotate independently of each other. On the other hand, the drive plates and the driven plates are usually splined to the case and the output shafts, respectively, so that they may be assembled sequentially so as to be rotatable integrally with the latter. Those plates, therefore, are capable of sliding in the axial direction. This sometimes causes the driven plates to get into the clearance between the output shafts to thereby lock the output shafts to each other, preventing them from performing smooth differential movements.
Generally, guide rings are interposed between the nearby drive or driven plates in order to space the plates by substantially constant distances from each other. Therefore, a possible implementation against the axial movement of the driven plates as stated above is positioning the driven plates in the axial direction by means of the guide rings. However, because scattering is unavoidable in the thickness of the plates and that of the guide rings and because the position of the clearance between the output shafts is effected by errors inherent in assemblage and others, it is impossible to prevent the driven plates from being located between the output shafts by simply restricting the axial movement of the driven plates.
Another problem is that the output shafts which face each other cannot be supported at their inner ends and, therefore, are unavoidably cantilevered within the case. In such a support structure, the inner end portions of the output shafts are apt to oscillate, failing to rotate themselves and transmit torque smoothly.
With regard to the fluid, it is filled in the power transmission chamber through an opening, which is formed through the case, after the assemblage of the differential device. The power transmission characteristic of the differential device is greatly effected by the ratio of the amount of fluid packed in the chamber to the volume of the chamber, i.e. packing ratio. It is necessary, therefore, that the packing ratio be adjusted delicately to an optimum one. Such delicate adjustment is very difficult, however, partly because the fluid usually comprises silicone oil whose viscosity is significantly high and partly because a number of plates are densely packed in the power transmission chamber. Moreover, because the opening adapted to pack the fluid has to be sealed, it is not easy for one to finely adjust the packing ratio of the fluid while observing the operating conditions of the differential device.